leetcode 159. Longest Substring with At Most Two Distinct Characters

Given a string s , find the length of the longest substring t  that contains at most 2 distinct characters.

Example 1:

Input: "eceba"
Output: 3
Explanation: t is "ece" which its length is 3.

Example 2:

Input: "ccaabbb"
Output: 5
Explanation: t is "aabbb" which its length is 5.

解題思路：

class Solution {
public:
    int lengthOfLongestSubstringTwoDistinct(string s) 
    {
        if(s.empty()) return 0 ;
        
        int len = s.size() , posi = 0 , posj = 0 , res = 0;
        unordered_map<char , int> fromItoJ ;
        
        fromItoJ[s[0]]++ ;
        while(posi <= posj && posj < len)
        {
            while( posj < len && fromItoJ.size() <= 2)
            {
                posj++ ;
                fromItoJ[s[posj]]++;
            }
            
            if(posj - posi > res) res = posj - posi ;
            
            while(posi <= posj && fromItoJ.size() > 2)
            {
                fromItoJ[s[posi]]-- ;
                if(fromItoJ[s[posi]] == 0) fromItoJ.erase(s[posi]) ;
                posi++ ;
            }
        }
        
        if(posj - posi > res) res = posj - posi ;
        
        return res ;
    }
};